Veneering both sides of plywood ??


Most articles and TV woodworking shows insist that you MUST laminate both sides of any material when you laminate. What is the rationale for laminating BOTH sides of plywood? If I am using Baltic ply (or apple ply) and I apply a thin (1/8"-1/4") veneer to only one side only, what damage could arise. How does a sheet of 13 ply Baltic know another ply has been attached.??
30-40 years ago we were able to purchase "lumber core" plywood. Certainly a lumber core construction would require that both sides be laminated for obvious reasons.
John
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The combination of the veneer and the glue (wetness) when it dries has the tendency to cause any wood it's attached to to cup if applied to only one side.
Agreed that plywood with its layers applied at cross angles minimizes the changes of cupping, but it can still happen and I've seen it happen.
Depending on how the 13 ply sheet of plywood is fastened down or if it's purpose is panelling for a cabinet will go a long way to determine if you really need to veneer both sides.
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The reason is to keep the plywood from warping. Yes plywood is many veneer ply's but they are all glued at the same time. Introducing a lot of moisture from glue to one side will slightly warp the piece. Its more IMHO a rule of thumb as I have never practiced this with a more likely to have problerms, plascic laminate.

I can still purchase lumber core today.
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About 100 million square feet of MDF, chipboard and ply are laminated each year with some form of plastic laminate and used as kitchen countertops. Needless to say, this is an accepted standard without question. In most cases the glue being used is some form of contact cement and would not add significant moisture to the sandwich.
Since I am about to start some serious veneering projects (amateur at best) I would like to understand the pros and cons of the process. I am considering applying several different wood veneers (homemade) to1/2 inch apple ply and using it as a decorative top to a small table. (16" x 18"). I have not decided on the adhesive....contact vs yellow. Advantages to both, with size of the project being one major factor.
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Kitchen countertops are glued or screwed into place. In those cases, there wouldn't be enough play for anything to warp or cup much. As well, most laminated countertops are are commercially made, at a guess in a fairly controlled environment.
Your applying veneer to a tabletop while veneering only one side is courting disaster, especially considering it's only 1/2" thick. But, it's your choice how you proceed.
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John Eppley wrote:

So? Plastic laminate doesn't warp.
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dadiOH
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I still do not understand the warping problem. Of course plastic doesn't warp. Of course a .025" oak laminate should not affect a 13 ply apple plywood.
As I understand the physics of the problem, applying any new material to a substrate causes uneven absorption of moisture on one side thus causing warp, if and only if, the absorption rate is significantly different on both surfaces. Otherwise, no effect.
If the sandwich is mechanically fastened and also has a protective finish I fail to understand the basic physical problem since any moisture absorption is contained and controlled. Again, this is a long term problem, if any. I can readily understand the possible affect of water soluble glues being applied liberally to a unstable sandwich. Contact cement is another story. Now if I were to border this small sandwich with a hardwood edging I believe the inert forces should provide additional containment.
Since I am attempting a new (to me) process, I want to learn from the first hand results experienced by other woodworkers. As a retired physicist and long term woodworker I have seen many "old wifes tales" come and go. "never sharpen tools using sandpaper", "never use steel wool on wood". Additionally, I have visited many websites offering veenering materials, supplies and advice. They offer contradictory and sometime confusing advice.
I'm still in a learning mode. Keep it coming.
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You will notice the plywood has plies the grain alternating for each layer .You will also notice that each ply is the same thickness . so if you apply a veneer to one side then you probably will not have a problem provides it is the same thickness as the plies and the it is laid crossgrain and on both sides [keeping the number of plies an odd number] One other thing you will notice the number of plies is always an odd number. If you veneer in the same direction as the plywood grain then you will creat an imbalance in surface stresses causing the panel to move until the stresses in the combination panel [ply plus veneer] are relieved ...
similarly if only veneer one side the panel becomes imbalances [ie more plies one side of the center lpy than the other
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One word:
Expansion coefficient.
Okay..those were 2 words...
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John Eppley wrote:

1. Dampen thin material with glue
2. Thin material expands
3. Adhere thin material to something else
4. Thin material dries
5. Thin material shrinks back to former size
6. "Something else" is bent all to hell.
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Veneer wouldn't have the same 'pull' as a sheet of plastic (HPL) laminate, but unbalanced laminating is asking for trouble. Baltic Birch already has the ability to warp all on it's own, at least the Russian/Finnish products we get here in Canada, so as a substrate for laminate, I think I would prefer high density particle board or MDF. Laminating onto plywood is not a good idea.
Tear tests have shown that high density particle board hangs on to laminate a lot better than the first little ply of a sheet of plywood...which makes sense when you think about it. (Of course, the quality of the plywood will make a huge difference, but why laminate a high quality plywood?) A contact cement joint, properly done (there are many improper ways) will not easily come apart unless exposed to high heat or certain solvents. Wilsonart WA3000 PVA will not let go no-how-noway, but it is a bear to work with. ( 5 minutes open time) http://www.wilsonart.com/adhesives/techdata/techdata.aspx http://www.wilsonart.com/adhesives/manufacture/manufacture.aspx
Unbalanced (without a second sheet on the opposite side) countertops will warp unevenly, even though the look of it is minimized when attached to cabinets, I have seen an unbalanced countertop pull an end-gable beside a dishwasher off the floor. Changes in humidity wreak havoc on porous materials. Period.
Most laminate manufacturers, such as Formica, Wilsonart (my personal favourite bunch of people to work with) make a specific material for use as a 'backer', 'balance sheet'. It is basically phenolic impregnated kraft paper, cheap as dirt and works just like laminate. When applied to the opposite side, in the same way as the 'good' side, chances of warping are virtually eliminated.
Using a balance sheet also takes away the particle board smell which often permeates throughout kitchen cabinets (Off-gassing formaldehyde etc.) reducing the chance you might run into a client with low tolerance for that kinda irritant.
I have also read that some marquetry/inlay/veneer artists try to chose substrates which live and breathe like their veneers, such as poplar.
HTH FWIW
Rob www.topworks.ca
------------------------------------------------------ Peace doesn't come from raising sheep. It comes from killing wolves.
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best)
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Contact cement - if the veneer is thin and you finish with lacquer, the thinner in the lacquer may absorb thru the veneer and release the contact cement. A wash coat of shellac will prevent this. Yellow glue - I've had success applying a thin coat to both surfaces and letting dry to a tack, then applying the veneer with a warm iron. Never tried it on anything large tho.
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Great advice.......I had considered MDF as my underlay but thought that would be "too easy". I had strong feelings about the integrity of the plywood available today. Including Finnish/Russian /Apple plywood.
I investigated the expansion coefficient of the woods I had been considering and found Maple to be within the limits of the imported plywood. Having said that, I have experienced some nasty warps with the imported and apple ply. Makes me doubt the data.
MDF sounds like the best substrate and any backing veneer should have similar expansion coefficients or the whole sandwich is in jeopardy. My "homemade" veneers will be at least 1/16" so I should choose my backing juduciously.
OK, now back to the "sawdust" factory.
Thanks for the advice. Much appreciated.
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